It is known that in circular knitting machines use is made of one or more selection assemblies, arranged around the needle cylinder before each feed or drop, each assembly being constituted by an electromagnet which is of maximum width equal to the spacing of the needles and which is interposed between two permanent magnets. A pressure cam is provided which is arranged in the region of the magnet and is upstream of said electromagnet in the direction of advance of the cylinder and which has the function of pushing against the latter an oscillating selector which is accommodated in a corresponding groove of the cylinder and is associated with a pusher jack. It is widely known to control the electromagnet using pulses, according to an electronic program synchronized with the movement of the needle cylinder. This is done by means of passing, interrupting or reversing the direction of the current, in order to make it possible for the permanent magnets to selectively attract the oscillating selector and to avoid the heel of the pusher jack engaging a control cam and bringing about the deactivation of operation of the corresponding needle by weakening the electromagnets magnetic field, it is possible for the permanent magnets to push back said oscillating selector, causing the raising of the heel of the pusher jack by the same control cam and the putting into service of the corresponding needle.
It is known that in the attempt to overcome the limitations of speed and of needle fineness currently imposed, it has been proposed to replace the oscillating selector jack in the vertical groove of the needle cylinder by an elastic selector jack. Such an elastic selector jack is arranged horizontally in a corresponding radial groove of a horizontal collar fixed around the needle cylinder. The electromagnet and the relevant permanent magnets are then arranged close to the head of the horizontal selector jack which is placed in contact with the corresponding permanent magnet by the foot of the same pusher jack.
However, this known system still has the problems associated with the selection of the selector jacks by means of an electromagnet of very small dimensions and such problems increase with an increase in the speed of the cylinder and in the fineness of the needles. These problems are as a result of the precariousness of the equilibrium of the forces in play and the variations, which are difficult to evaluate, in the various factors such as friction, residual magnetism, induced magnetism and elastic forces which vary as a result of the differences in the thermal treatments and in the dimensions or bending of the different pieces. Therefore, and by way of example, in a circular machine with 400 needles, the theoretical equilibrium between the weight of a selector jack and the force of attraction which can be obtained using an electromagnet is reached at speeds which are no greater than 500 revolutions/min. and this is incompatible with the speeds, fineness and reliability currently required by the industry which uses circular machines for stockings, especially women's stockings.